In conventional variable air volume (VAV) systems which are capable of variable output by varying the fan speed, positioning the inlet guide vanes or by controlling the pitch of an axial fan, fan control is achieved through the use of a single pressure sensor which is located somewhere in the main duct line. The pressure sensor sends an input directly to the fan control such as a variable speed drive of an air handler unit to cause the speed to increase, decrease or remain constant. Thus, if the pressure at the sensor location is too high, a "decrease" signal is sent and, if it is too low, an "increase" signal is sent. The location of the sensor is carefully chosen in order that the pressure sensed is most closely representative of the overall system pressure distribution over the whole load range. Ordinarily, the sensor location chosen is about two thirds of the way down the main duct as measured by distance or total pressure drop. Space temperatures are not used to control fan speed. Instead, the sensed pressure is fed back to the speed changer through a parallel summing junction/function generator control process. The speed changer alters fan speed and thereby fan outlet pressure. There is a pressure loss in the remaining main/common duct downstream of the pressure sensor. The duct system splits up the flow, losing more pressure in the branches, is delivered to a VAV terminal control loop which imposes a controlled pressure loss and ultimately influences the space temperature. Thus, although temperature is the variable to be controlled, all of the controls are pressure responsive, and a single pressure sensor is used. So, even if the pressure sensor is located at the point where the sensor will give the "best" representation of the overall duct pressure distribution at design flow, when there is off design flow and when the VAV terminals on one side of the duct system close off, however, the "best" spot changes to another location. As a result, the sensor may be inaccurate in its representation of duct pressure distribution under off-design conditions as well as those due to normal changes during the day.
Independent of how well placed it is, a single sensor provides a single reading and it is not an average or otherwise influenced by measurements taken at other locations. Because of this, a single reading cannot give the variable speed control a good view of what is required in the duct under all conditions. Since a full view is not provided, this approach is inherently inaccurate under at least some operating conditions. In addition, in order to compensate for inaccuracies, the building operator generally sets the one pressure sensor at a "fat" setting so as to put a comfort factor in and thereby avoid occupant complaints but at the expense of greater energy usage.